Nitrogen dioxide (NO2) is a secondary by-product of the combustion processes found in almost all vehicles and power plants. The high-temperatures and very reactive chemical species found in combustion form nitric oxide (NO) from oxidation of N2 and fuel nitrogen. This NO then rapidly oxidizes in the atmosphere to form NO2.
There is substantial scientific evidence that links short-term NO2 exposure (30 minutes-24 hours) to respiratory illness (e.g. asthma and possibly emphysema) and cardiovascular mortality, with a strong correlation between NO2 levels and hospital admissions for respiratory problems. NO2 susceptibility is particularly high for at-risk (e.g. children, elderly, and those with respiratory issues) and underserved communities. Additionally, NO2 measurements serve as a general indicator of total nitrogen oxide (NOx) concentrations. NOx, in turn, reacts with atmospheric constituents (e.g. ammonia, water and volatile organic compounds to form small particulates and ground-level ozone respectively, which can lead to premature death, lung tissue damage, and respiratory diseases.
Due to the adverse effects of short-term NO2 exposure and recent scientific verification, the EPA has established an annual, average NO2 standard of 53 ppbv (=100 μg/m3), strengthened the primary, 1-hour NAAQS for NO2 to 100 ppbv, and is making changes in the air quality monitoring network to focus on locations with maximum NO2 concentrations [Environmental Protection Agency, 40 CFR Parts 50 and 58, Feb. 9, 2010, “Primary National Ambient Air Quality Standards for Nitrogen Dioxide; Final Rule,” Federal Register 75 (26) 6473]. The monitoring network will now include analyzers near major roadways in urban areas, community-wide measurements in large population centers, and additional sensors in particularly vulnerable communities. Future NO2 standards may be even more stringent, with the World Health Organization recommending an annual compliance standard of 21 ppbv (=40 μg/m3), and the California Air Resources Board approving a level of 30 ppbv.
Likewise, sulfur dioxide (SO2) is a major atmospheric pollutant. Once in the atmosphere it is oxidized in the gas phase and in clouds to form sulfuric acid (H2SO4) and leads to the production of aerosol sulfate, the deposition of which contributes to acidification of the surrounding ecosystem. Typical concentrations have fallen in recent years to less than 50 μg/m3 (18.8 ppb) in most areas, but in certain conditions can exceed 24-hour average levels of 100 μg/m3 (37.6 ppb) with peak concentrations over 500 μg/m3 (188 ppb) at times. The U.S. Environmental Protection Agency has set a primary standard of 75 ppb for the average 1-hour daily maximum concentration of sulfur dioxide [75 FR 35520, Jun. 22, 2010]. The World Health Organization has set a 24-hour average interim target level (IT-1) for sulfur dioxide of 125 μg/m3 (47.0 ppb) and a guideline level of 20 μg/m3 (7.5 ppb).
Currently, the EPA Test Method approved for NO2 detection (e.g. Method 7E) involves catalytic reduction followed by chemiluminescence. The ambient air sample is first reacted with ozone to convert nitric oxide into excited nitrogen dioxide:NO+O3→NO2*+O2.The excited NO2* then deactivates via visible luminescence that can be detected by a photomultiplier tube. Quantification of the emitted intensity provides a measurement of the NO concentration. The air sample is then passed over a hot, catalytic surface (e.g., Mo at 375° C.) to reduce NO2 to NO, and the chemiluminescence detector sees an additional signal due to the increase of NO. The concentration of NO2 can then be determined from the difference in luminescence before and after catalytic reduction.
There are several disadvantages to this measurement strategy. Foremost, the hot catalytic surface also converts a variety of other ambient nitrogen-containing, air species (e.g. peroxy acetyl nitrate, alkyl nitrates, HNO3 . . . ) into NO. Therefore, chemiluminescence detectors substantially overestimate NO2 concentrations (by as much as a factor of 2), a critical limitation as compliance standard levels are further decreased. EPA has recognized this problem [U.S. EPA, “Integrated Science Assessment for Oxides of Nitrogen—Health Criteria,” EPA/600/R-07/093] and suggested that such detectors are better suited to detect the total concentration of nitrogen oxides (NOx).
A number of alternative strategies have been proposed to more accurately measure ambient NO2 concentrations:
Photolytic Chemiluminescence
Instead of using a hot catalyst, photolysis near 300-400 nm can be used to convert NO2 into NO (and ozone) [M. Buhr, “Development and Field Testing of a Small, Efficient, Photolytic Converter for Measurement of Ambient Nitrogen Dioxide,” 2008 National Air Quality Conference, Apr. 8, 2008]. This method is much more specific to NO2 and avoids interferences from other atmospheric, nitrogen-containing compounds. However, the conversion efficiency is not well-characterized and can vary over time as the photolysis source and gas residence time change. Moreover, the photolysis source typically operates at a relatively high radiant flux, thus limiting its lifetime to ˜5000 hours (208 days of continuous operation). Finally, though the instruments are relatively cost-effective (e.g. ˜$20 k with photolytic converter), they require significant labor (e.g. calibration and maintenance) and toxic consumables (e.g. O3, NO).
Luminol Chemiluminescence
Alternatively, luminol can react with NO2 to form an excited state that subsequently luminesces [J. S. Gaffney et al., “Aircraft Measurements of Nitrogen Dioxide and Peroxyacyl Nitrates Using Luminol Chemiluminescence with Fast Capillary Gas Chromatography” Environ. Sci. Technol. 33 (19) 1999, 3285]. Although this technique does not require conversion of NO2 into NO, it is not linear at low NO2 concentrations and exhibits cross-interferences with peroxy acetyl nitrate.
Mid-Infrared Tunable Diode Laser Absorption Spectrometry (TDLAS)
A tunable laser operating near 6.23 μm can also be used to very accurately quantify ambient NO2 concentrations using absorption spectrometry. Although this technique is very selective and accurate, it is prohibitively expensive (e.g. $80-$120 k retail) and insufficiently robust for compliance monitoring applications.
There are also a host of more complicated, laboratory-based measurement techniques for NO2, including electron spin resonance [D. Mihelcic, P. Musgen, and D. H. Ehhalt, J. Atmos. Chem. 3, 341 (1985)], laser-induced fluorescence and differential optical absorption spectrometry [S. T. Sandholm, J. D. Bradshaw, K. S. Dorris, M. O. Rodgers, and D. D. Davis, J. Geophys. Res., [Atmos.] 95, 10155 (1990)]; however, these technologies are still primarily limited to laboratory instrumentation.
More recently, visible cavity-enhanced optical absorption methods have been used to accurately quantify ambient NO2 concentrations using both lasers and light emitting diodes (LEDs). Cavity ringdown spectroscopy (CRDS) near 400-532 nm has been used by a variety of research groups to achieve a measurement precision of better than ±40 ppt, with minimal interferences from other atmospheric species [P. Castellanos et al., Rev. Sci. Instrum. 80 (2009) 113107; and H. Fuchs et al., Environ. Sci. Technol. 43 (2009) 7831]. Similarly, Integrated Cavity Output Spectroscopy (ICOS) [J. Langridge et al., Analyst 131 (2006) 916] and Cavity Attenuated Phase shift Spectroscopy (CAPS) [P. L. Kebabian et al., Environ. Sci. Technol. 42 (2009) 6040] have attained comparable results.
Recently, cavity ring-down spectroscopy has been used to detect sulfur dioxide down to 3.5 ppb concentrations, employing a frequency-doubled dye laser at 308 nm wavelength as a coherent light source for the optical cavity [David S. Medina, Yingdi Liu, Liming Wang, and Jingsong Zhang, “Detection of sulfur dioxide by cavity ring-down spectroscopy”, Environ. Sci. Technol. 45(5) (Mar. 1, 2011) 1926-31].